1. Field of the Invention
The present invention relates to a method for determining the flatness of a material strip, as well as to a device for performing the method.
2. Background Information
Undesired unevennesses extending in travel in the longitudinal direction, as well as in the transverse direction thereto are formed in a metal sheet produced in the form of a material strip during the cold and hot rolling of metal sheets. These unevennesses cause the material strip to be deflected to various extents perpendicular to the surface, thus spoiling the flatness, and leading to different strip elongations for several longitudinal portions of the material strip which are disposed transverse to the longitudinal direction. It is therefore necessary during the rolling of a metal sheet to monitor the flatness of the produced material strip and, if deviations from flatness are detected, to influence the conditions of the rolling process.
The value of strip elongation is measured in xe2x80x9cIxe2x80x9d units, where one I units means a relative length change of 10xe2x88x925, which corresponds, for example, to 10 xcexcm per meter.
Several methods for measuring flatness are known.
A first method comprises scanning the surface of the material strip by means of a pulsed laser beam, with which a grid of points and their associated distance from the laser light source is recorded. The results are used to determine the deflection of the material strip and thus the flatness.
In a second method, a geometric pattern such as a striated pattern is projected onto the surface by means of an optical imaging device. This pattern is monitored by a camera. Surface deflections distort the pattern, and the magnitude of the distortion provides a measure of the flatness.
The two methods described in the foregoing work on contactless principles, and so they are used preferably in the hot-rolling process. The ambient conditions, however, necessitate frequent maintenance of the optical components, especially during hot rolling. In both methods, moreover, a measuring device must be set-up in addition to the devices normally used for measurement of strip thickness profiles. These devices usually operate with high-energy electromagnetic radiation.
A third method uses a plurality of pressure sensors, which are disposed side-by side, roll along with the material strip and are in contact with the material strip. Different deflections lead to different pressures, and so the measured pressures can be evaluated as a measure of flatness. The disadvantage of this method lies in the mechanical contact of the individual pressure sensors with the material strip, and so, especially in the case of the hot-rolling process, the method cannot be used because of the high temperatures. Even in cold rolling, however, the method suffers from the disadvantage that the mechanical contact leads to wear.
Finally, methods and devices are known that use high-energy electromagnetic radiation such as X-rays or gamma rays to measure strip thickness transverse profiles, as well as the strip contour, or in other words, the shape and position of the material strip over the width. Using this measuring method, however, it has not yet been possible to determine the flatness of the material strip.
It is emphasized that unevennesses which can also be measured by means of the method describe hereinafter can occur not only in material strip produced from metal sheets, but also in material strips from other materials. Thus the term material strip, rather than metal strip is used in general hereinafter.
The technical problem underlying the present invention is to specify a method and a device for determining the flatness of a material strip, in which device and method the strip elongation is calculated from the values of strip contour.
The present invention thus concerns a method for determining the flatness of a material strip, the material strip predefining a longitudinal direction, comprising:
recording measured values at a plurality of measurement points by at least two radiation sources and a plurality of detectors, the measurement points being disposed transverse to the material of the strip and being sensed by at least two detectors, each of which detects radiation at various solid angles,
moving the material strip in the longitudinal direction relative to the radiation sources and the detectors, and rows of measured values substantially encompassing all measurement points are recorded at each of several given intervals,
calculating the slope of the material strip for each recorded measurement point from the measured values of the detector pairs,
calculating the wavelength and phase of slope changes for successive rows of measured values at a known relative velocity in the longitudinal direction,
calculating at least one extremum and the respective associated closest row of extreme measured values from the wavelength and phase,
calculating the transverse contour by summing the slope values of the rows of extreme measured values, and then determining the amplitude of the transverse contour, and
calculating the strip elongation from the wavelength and amplitude of the contour.
The present invention also relates to a device for determining the flatness of a material strip, the material strip predefining a longitudinal direction, comprising:
at least two radiation sources, which are disposed transverse to the longitudinal direction and spaced apart from each other;
a plurality of detectors, which are disposed transverse to the longitudinal direction at a distance from each other and from the radiation source, the material strip being disposed between the radiation sources and the detectors, each of at least two detectors being oriented towards two different radiation sources and forming a detector pair or pairs, and axes formed respectively by a detector together with a radiation source intersecting each other substantially in the region of the material strip and thus predefining measurement points; and
means for evaluation of measured values which are recorded by the detectors, the evaluation means calculate from the measured values the slope of the material strip at the measurement points and therefrom the flatness of the material strip.
The technical problem described in the foregoing is solved by the method according to the present invention, wherein measured values are first recorded at a plurality of measurement points by means of at least two radiation sources and a plurality of detectors. The said measurement points are disposed such that they lie transverse to the longitudinal direction and spaced apart from each other in the material of the strip.
The measurement points are sensed individually by at least two detectors, each of which detects radiation at various solid angles. At any time, therefore, one detector is oriented towards one of the at least two radiation sources and the other detector is oriented towards the other radiation source. Thus those volume elements of the material strip through which there passes the radiation sensed by the detectors can be regarded as the measurement points.
Furthermore, the material strip is moved in the longitudinal direction relative to the radiation sources and the detectors. Rows of measured values substantially encompassing all measurement points are recorded at each of several given intervals. The slope of the material strip is then calculated for each recorded measurement point from the measured values of the detector pairs. Thus there is obtained a grid of measured values and associated slope values extending over a given region of the material strip.
Knowing the velocity of the material strip in the longitudinal direction relative to the radiation sources and detectors, it is then possible to calculate, for successive rows of measured values, the wavelength and phase of the slope changes, which changes characterize the flatness. In this context, the wavelength is to be understood as the distance between each of two successive regions with the same deflection up or down.
Furthermore, there is calculated from the wavelength and phase at least one extremum, for which it is true that the magnitude of the slope component in the longitudinal direction is minimal. This ensures that the slope values have substantially only a transverse component, which characterizes the deflection of the material strip in the transverse direction; this deflection being responsible for the strip elongation.
At each extremum there is calculated a row of extreme measured values, which in each case represents the row of measured values located closest to the extremum, since the rows of measured values are distributed discretely and not continuously over the material strip. In this way there is obtained the most accurate possible approach to the extremum, and the row of extreme measured values contains the information necessary for determination of the transverse contour.
The transverse contour is calculated by summing the slope values of the row of extreme measured values, and the amplitude of the unevennesses at the extremum is determined for each measurement point from the transverse contour. Finally the strip elongation is calculated from the wavelength and the amplitude of the transverse contour. In fact, a strip elongation can be calculated for each length element of the material strip containing a succession of corresponding measurement points in the longitudinal direction.
According to the present invention, therefore, it has been recognized that the unevennesses in the material strip can be determined across and along the material strip on the basis of the variable absorptions of radiation. Furthermore, the components in the longitudinal direction and in the transverse direction thereto, contained in the respective slope values, are advantageously evaluated independently of each other.
Preferably the radiation intensity attenuated by the material strip is measured by the detectors. In this case the degree of attenuation is a measure of the thickness of the material strip through which the radiation has passed.
It is further preferred that the measurement points cover substantially the entire width of the material strip. Thereby it is possible to examine the entire width of the material strip with one row of measured values. Reciprocating linear movement of the radiation sources and detectors transverse to the longitudinal direction is then unnecessary, although the number of detectors is relatively large.
The accuracy of the method can be further improved by additionally moving the detectors forward and back across the material strip through an amplitude in the range of the distance between two detectors while the material strip is moving in the longitudinal direction. Thereby the regions between each two detectors can also be sensed, and so regions of the material strip that would otherwise not be sensed can be sensed.
Furthermore, the measurement points can be combined in measuring channels, each of at least two measurement points. Preferably the measuring channels each encompass substantially the same number of measurement points, and the values of the slopes are calculated for each measuring channel. It is further preferable to calculate the strip elongation separately for each measuring channel. Thereby the information of neighboring measurement points is combined, thus achieving an improved signal-to-noise ratio. It is also possible to combine all measurement points in one measuring channel or halves of the measurement points in each of two measuring channels. The size of the measuring channels can be adjusted as a function of the quality of the measured values.
In a further preferred embodiment the wavelength and phase of the unevennesses are calculated by means of a Fourier transform. It is also possible, however, to use other mathematical methods with which wavelength and phase of the unevennesses can be calculated.
As explained hereinabove, a row of extreme measured values is determined for each extremum. Preferably the strip contour in the region of the extremum is calculated from the data of the row of extreme measured values and of at least one further adjacently disposed row of measured values by arithmetic averaging. Thereby the signal-to-noise ratio is also improved. In particular, those two rows of measured values between which the calculated extremum lies are used for evaluation.
Furthermore, the strip elongation is preferably calculated in I units by means of the formula                     (                              Amplitude            ·            π                    Wavelength                )            2        ·          10      5        ,
wherein the amplitude and the wavelength are in units of meters. For this purpose it is assumed that the unevennesses of the material strip are sinusoidal. This calculation can also be simplified by using a triangular form as an approximation, so that the strip elongation can be determined by a simple geometric calculation.
In the course of the method, a plurality of rows of measured values is needed in order to determine the wavelength and phase of the unevennesses. It is therefore possible to record rows of measured values for a given first strip length at the beginning of the measurement (at a starting period of time for measuring so as to sample enough data to be able to carry out the method), and then to evaluate these rows for a first time. Thereafter, or in other words, after the first given strip length, the measured values for a smaller, second given strip length are recorded, and then the measured values most recently recorded over an entire first strip length are evaluated, and so on. In other words, rows of measured values collected over a portion corresponding to the first strip length are always evaluated for determination of the strip length.
For example, measured values at intervals of 10 cm each are first recorded over a strip length of 10 meters. Thus initial evaluation results are obtained after the first 10 meters. Thereafter a further 2 meters of strip length are surveyed and the most recently measured 10 meters are evaluated. Thereby a moving average is obtained within the evaluation results.
The technical problem described above is solved also by the use of the device according to the present invention for measurement of the strip thickness profile of a material strip in order to determine the flatness. This device is provided with at least two radiation sources, a plurality of detectors and means for evaluation of the measured values recorded by the detectors. The detectors are disposed at a distance from each other and from the radiation sources, the material strip being disposed between the radiation sources and the detectors and moved in the longitudinal direction relative thereto. The detectors generate measured values at the measurement points disposed in the material strip, and the evaluation means calculate from the measured values the slopes at the measurement points and therefrom the strip flatness.
Thus it is possible for the first time to use, for measurement and checking of the flatness of the material strip as well, a device that heretofore has existed exclusively for measurement of the strip thickness profile. The technical complexity is therefore considerably reduced on the whole, since none of the separate devices necessary for performing the aforesaid methods known from the prior art are needed. Since the determination of flatness can be achieved with an already existing device for measuring the strip thickness profile of a material strip, the present invention can also be used for retrofitting existing devices, because the method according to the present invention represents substantially a detailed analysis of the measured values obtained heretofore.
A practical example of the present invention is depicted in more detail hereinafter with reference to the drawings.